The structural changes of spinel Li1.02Mn2O4 and Li1.02Co0.11Ni0.04Mn1.85O4 cathode materials have been studied by synchrotron powder X-ray diffraction and differential scanning calorimetry (DSC) measurements. The results show that spinel Li1.02Mn2O4 undergoes a phase transition from cubic (Fd3m) to orthorhombic symmetry (Fddd) at T = 285 K. However, substitution of a small amount of Co3+ and Ni3+ ions suppresses phase transition and the cubic phase is maintained at low temperature due to a decrease in the concentration of Jahn-Teller active Mn3+ ions. (C) 2003 Elsevier Science B.V. All rights reserved.